tun: annotate access to queue->trans_start

tun: annotate access to queue->trans_start

Commit 1db7b89a838a ("net: annotate accesses to queue->trans_start")
introduced a new helper, txq_trans_cond_update, to update
queue->trans_start using WRITE_ONCE. One snippet in drivers/net/tun.c
was missed, as it was introduced roughly at the same time.

Fixes: 1db7b89a838a ("net: annotate accesses to queue->trans_start")
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20220412135852.466386-1-atenart@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>